One of the methods of electronics mounting for mounting a semiconductor package on a printed circuit board, or electrically connecting respective conductor circuits on two printed circuit boards to each other and coupling or fixing both the printed circuit boards to each other is a method using a film-shaped anisotropic conductive film (see, Japanese Laid Opened Patent Publication Nos. JP-H06-102523-A2 (1994), JP-H08-115617-A2 (1996), etc.).
When a semiconductor package is mounted, a semiconductor package having a plurality of bumps arranged on its mounting surface to a printed circuit board to form a connecting portion, and a printed circuit board having a plurality of electrodes arranged with the same pitch as the pitch of the bumps in its region where the semiconductor package is mounted to form a connecting portion are prepared, and the semiconductor package and the printed circuit board are thermally bonded in a state where the connecting portions are opposed to each other and an anisotropic conductive film is interposed therebetween while being aligned with each other such that the bump and the electrode, in each of pairs, in both the connecting portions are overlapped with each other in the plane direction of the film.
When printed circuit boards are connected to each other, two printed circuit boards respectively having pluralities of electrodes arranged therein with the same pitch at their connecting positions to form connecting portions are prepared, and are thermally bonded in a state where the connecting portions are opposed to each other and an anisotropic conductive film is interposed therebetween while being aligned with each other such that the respective electrodes, in each of pairs, in both the connecting portions are overlapped with each other in the plane direction of the film.
The anisotropic conductive film generally has a structure in which a powdered conductive component is dispersed in a film having heat-sensitive adhesive properties including a binding agent such as thermoplastic resin or curable resin.
In the anisotropic conductive film, in order to prevent the occurrence of such short circuit in the plane direction of the film that the bump and the electrode or the electrode and the electrode, in each of the pairs, which are arranged with each other in the plane direction of the film are short-circuited with the bump and the electrode or the electrode and the electrode in the adjacent pair, a filling factor, found by the following equation (1), of the conductive component such that a conductive resistance in the plane direction (referred to as an “insulating resistance”) is increased:
                              filling  factor  (vol.  %)                =                                            (volume  of  conductive  component)                                      (total  volume  of  solid  contents)                                ×          100                                    (        1        )            In a case where the film is formed by the conductive component and the binding agent as solid contents, as described above, the total volume of the solid contents in the equation is the total of both the volumes of the conductive component and the binding agent.
The anisotropic conductive film is compressed in the thickness direction by heating and pressurization at the time of thermal bonding, so that the filling factor of the conductive component in the thickness direction is increased, and the conductive components are brought in close proximity to or into contact with each other to form a conductive network. As a result, a conductive resistance in the thickness direction (referred to as a “connecting resistance”) is reduced. In this case, however, the filling factor of the conductive component in the plane direction of the anisotropic conductive film is not increased. Therefore, the plane direction maintains an initial state where the insulating resistance is high and the conductivity is low.
Thus, the anisotropic conductive film has anisotropic conductive properties in which the connecting resistance in the thickness direction is low and the insulating resistance in the plane direction is high. The anisotropic conductive film allows,                while preventing the above-mentioned short circuit in the plane direction of the film from occurring, to maintain an electrically independent state for each of the bump-electrode pairs or the electrode-electrode pairs,        the bumps and the electrodes or the electrodes and the electrodes in all the pairs, which are respectively arranged with each other in the plane direction of the film, to be simultaneously conductively connected to each other.        
In addition thereto, the heat-sensitive adhesive properties of the film allows the semiconductor package to be fixed on the printed circuit board by thermal bonding or the printed circuit boards to be fixed to each other by thermal bonding.
The use of the anisotropic conductive film makes it easy to perform work for electronics mounting.
Various types of metal powders such as an Ni powder having an average particle diameter of several to several tens of micrometers and having a granular shape, a spherical shape, or a foil shape (a scale shape, a flake shape), resin powder whose surface is gold-plated, and so on are put to practical use as a conductive component included in the conventional anisotropic conductive film.
The conventional anisotropic conductive film generally contains the above-mentioned metal powder such that the filling factor found in the foregoing equation (1) is 7 to 10% by volume.
In the range of the filling factor, however, the value of the connecting resistance in the thickness direction after thermal bonding is not sufficient, so that the number of cases where it is required that the connecting resistance should be made much lower is increasing.
Therefore, it is considered that the filling factor of the metal powder serving as a conductive component is made higher than that in the above-mentioned range in order to further make the connecting resistance in the thickness direction lower than before.
In such a case, however, in the conventional anisotropic conductive film using the above-mentioned general metal powder, the insulating resistance in the plane direction of the film is also reduced, so that short circuit in the plane direction of the film easily occurs.
Since such a problem easily occurs, the conventional anisotropic conductive film cannot cope with the above-mentioned requirements unless the pitch of the adjacent bumps or electrodes composing the connecting portion is not less than 50 μm. In the present circumstances, the conventional anisotropic conductive film cannot cope with requirements of higher density mounting in the field of electronics mounting.
In recent years, the inventors have examined that in a probe card used for examining whether or not a semiconductor package chip such as a memory, an IC, an LSI (Large Scale Integrated Circuit), or an ASIC (Application Specific Integrated Circuit) is normally manufactured, one anisotropic conductive film is used in place of a lot of wirings used for respectively connecting a lot of fine contact probes mounted on a mounting substrate to electrodes provided on a circuit in a probe card main body. The inventors have considered that in such connection, the mounting pitch of the contact probes is approximately 100 to 200 μm in correspondence with the pitch of pads in the semiconductor package chip and therefore, even the conventional anisotropic conductive film can cope with the above-mentioned requirements.
That is, the probe card serves to achieve conduction by pressing the contact probe against the pad on the semiconductor package chip which has not been cut to a predetermined size, formed on a wafer, thereby connecting a circuit in the semiconductor package chip to an external examination circuit through the circuit in the probe card main body to examine the circuit. However, as the semiconductor package chip is miniaturized and highly integrated, the pad itself or the formation pitch thereof is miniaturized, or the number of pads is increased, the contact probe itself tends to be refined or highly integrated on the mounting substrate.
Particularly in recent years, a probe card in which a lot of very fine contact probes processed with a processing accuracy in micron units are mounted on a mounting substrate with the same pitch of 100 to 200 μm as the pitch of the pads in the semiconductor package chip, as described above, has been put to practical use.
In the probe card for examining several tens to several hundreds of semiconductor package chips formed on one wafer, however, several thousands of contact probes must be mounted on the mounting substrate. The number of wirings for connecting the contact probes and the probe card main body must be also the same as the number of contact probes. Therefore, the number of times of soldering of the wirings becomes enormous.
Therefore, the manufacture of the probe card and the management thereof at the time of use are significantly difficult.
Therefore, the inventors have examined that a lot of wirings and their soldering are replaced with one anisotropic conductive film. Even if the conventional anisotropic conductive film is simply used for another purpose, however, the following problems occur. Therefore, it has been found that practical applications thereof are difficult.
(i) When an internal circuit in a semiconductor package chip to be tested is short-circuited, a large current of not less than 1 A, may flow locally through the anisotropic conductive film at the time of the test. However, the conventional anisotropic conductive film does not consider response to such a large current. A current value to be allowed is only several ten milliamperes. When a large current flows by short circuit or the like, therefore, Joule heat is produced so that the temperature of the anisotropic conductive film locally rises. Therefore, the anisotropic conductive film may be fused.
(ii) The contact probe is very small and is liable to be destroyed, as described above. When the anisotropic conductive film is used for mounting the contact probe, that is, connection to an electrode, therefore, pressurization at the time of thermal bonding must be performed at a lower pressure, as compared with that in the case of the above-mentioned normal connection between the bump and the electrode or between the electrodes. When the contact probe and the electrode are connected to each other at a low pressure, however, a connecting resistance in the thickness direction cannot be reduced to a sufficiently practical level, which may cause inferior connection in the conventional anisotropic conductive film.
(iii) When the filling factor of a metal powder is increased in order to eliminate the inferior conduction, the insulating resistance in the plane direction is also reduced in the conventional anisotropic conductive film. Even if the pitch is 100 to 200 μm, the above-mentioned short circuit in the plane direction of the film, that is, short circuit between the contact probe and the electrode, in each of the pairs, which are arranged with each other in the plane direction of the film in this case, and the contact probe and the electrode in the adjacent pair may occur.
(iv) In order to examine a semiconductor package chip for a graphic board or a computer game and a high-speed semiconductor package chip such as a Ga—As device, at an operation speed actually used, a high-frequency signal must be used. When the filling factor of the metal powder is increased to eliminate the inferior conduction as in the item (iii), however, it is difficult to pass the high-frequency signal because the impedance of the anisotropic conductive film is increased, so that the anisotropic conductive film may be unable to be examined.
(v) The semiconductor package chip to be examined by the probe card is distributed over the whole surface of one wafer in many cases, as described above. Therefore, the substrate on which the contact probe is mounted and the probe card main body are formed to such large sizes as to cover the wafer. Consequently, the anisotropic conductive film for connecting the probe card must cover a significantly larger size than that for the conventional semiconductor mounting. Moreover, at the time of the above-mentioned connection at a low pressure, variations in the thickness direction such as warping of the large members must be absorbed over the whole surface, not to cause inferior connection, inferior conduction, and so on. However, it is difficult for the conventional anisotropic conductive film to cope with such requirements.